Liquid crystals (LCs) are matter in a state that has properties between those of conventional liquid and those of solid crystal. In a liquid crystal, the molecules may orient themselves in a variety of ways, some of which are described herein. The liquid crystals may exist in a nematic phase over a specific temperature range. In the nematic phase, the rod-shaped molecules have no positional order, but self-align in a long-range directional order with their longitudinal axes approximately in parallel to one another. For some liquid crystals, the molecules may take on a chiral orientation order in the nematic phase. At cooler temperatures, the molecules may form a conventional solidified crystal. At higher temperatures, the liquid crystals may exist in an isotropic phase, wherein the molecules have little to no long-range order.
For some liquid crystals, there exists a blue phase between the chiral nematic phase and the isotropic phase. Liquid crystals in the blue phase have a regular three-dimensional cubic structure of double-twisted cylinders with lattice periods of several hundred nanometers, and thus they exhibit selective Bragg reflections in the wavelength range of visible light that corresponds to the cubic structure.
A liquid crystalline blue phase is an optically isotropic phase which, upon application of an electric field, and based on the Kerr effect, becomes birefringent, and due to the refractive index distribution of the liquid crystal under the electric field, turns into an optically anisotropic phase. The blue phase of the liquid crystalline material is unlike other liquid crystal phases which typically switch from one anisotropic phase to another. If the liquid crystalline material in the blue phase is brought between two crossed polarizers, the transmittance increases with an increase in the voltage application. This operation requires application of a considerable voltage as the voltage is required to induce the birefringence which itself is highly dependent on the Kerr constant of the liquid crystal material and also on the strength of the electric field generated by the voltage.
Blue phases of liquid crystals are of interest for several applications, including fast light modulators and tunable photonic crystals. Blue phase liquid crystals may provide improvements in LCD screens, optical gratings, variable optical attenuators, photonic crystal lasers, and beam steering devices. However, there remain several drawbacks associated with the blue phase liquid crystal materials. Some of the drawbacks include the high operating voltage required to switch the blue phase from a dark to a bright state, relatively low transmittance, long relaxation times, and narrow usable temperature ranges over which the blue phase is stable.
Dual-frequency liquid crystals allow for faster response times, however, for combined blue phase dual frequency liquid crystals, the other issues still remain. Blue phase dual frequency liquid crystal compositions have a very narrow blue phase stabilization range. This is due primarily from the defects which inevitably form in the three-dimensional crystal lattice during molecular self-assembly of the double twisted spiral columns. The presence of the defects leads to instability of the whole system, and temperature ranges over which the blue phase is stable are generally less than about 12° C., and exist at temperatures above normal ambient temperatures.